The author would like to thank G. Carnovale, K. Y. Chan and G. Lusztig for discussions. The author is especially grateful to G. Carnovale and M. Costantini for pointing out some of the results in [6]. She would also like to thank the referee for very helpful comments. Research in this paper was partially supported by HKRGC grants 703405 and 703707 and by the Seed Fund for Basic Research from the HKU (Project code 10400942).